Light emitting diode based projection systems combine different colors of light produced by multiple LEDs. The different colors of light may be combined to produce a source of white light, or alternatively, an optical display, sometimes referred to as micro-display, e.g., liquid crystal on silicon (LCOS) or digital light processing (DLP), may be used to impart image information on the combined light or on each color of light before the light is combined.
FIG. 1 is a graph illustrating the spectral curves of a six primary LED projection system, i.e., a projection system that combines the light from six differently colored LEDs. FIG. 1 illustrates the spectral curves for Blue, Blue-Cyan, Cyan-Green, Green, Yellow-Orange, and Red colored light. As can be seen, in the areas 12 and 14, for example, there is substantial overlap in the spectra. The overlap is problematic as it leads to a loss of brightness in conventional systems.
FIG. 2 schematically illustrates two LEDs 22g and 22yo, which produce green and yellow-orange light, respectively, along with associated lenses 24g and 24yo. Conventionally, light from the LEDs 22g and 22yo is combined using a dichroic filter 26, which reflect some wavelengths and transmit other wavelengths. For example, the dichroic filter 26 may reflect wavelengths above 570 nm and transmits wavelengths below 570 nm. As can be seen in area 14 in FIG. 1, however, the spectral contribution from the Yellow-Orange light below 570 nm will be lost because it is transmitted through the dichroic filter 26, as indicated by the dotted lines 28 in FIG. 2, as opposed to being reflected by the dichroic filter 26 along the desired beam path 30. Thus, overlapping spectral contributions in multi-primary LED projection systems results in lost light when combining the different channels, and therefore a decrease in brightness.
FIG. 3 is a CIE 1931 color space chromacity diagram illustrating a six LED primary system. The space defined by the line 30 labeled “BCCGYR” is the gamut of color that can be matched by various combinations of the Blue, Blue-Cyan, Cyan-Green, Green, Yellow-Orange, and Red colored light. Conventionally, the multi-primary projection system combines the colors into Blue, Green and Red channels. Thus, the Blue and Blue-Cyan are combined into the Blue channel, the Cyan-Green and Green are combined into the Green channel and the Yellow-Orange and Red are combined into the Red channel. The spectral losses caused by the combination of these channels can be seen in areas 12 and 14 in FIG. 1. The space defined by the line 32 labeled “Combined” is the gamut of color that can be matched by various combinations of the Blue, Green and Red channels. As can be seen, conventional systems greatly limit the gamut of the color spectrum that can be produced.
Thus, conventional multi-primary LED light sources suffer from problems such as a loss of brightness and reduced color gamut. Improvements of multi-primary LED light sources, such as projection systems, are desirable.